Axial flow fan

ABSTRACT

The axial flow fan (1; 30) comprises a central hub (3; 33) a plurality of blades (4; 34) which have a root (5; 35), and an end (6; 36). According to one embodiment, the blades (4; 34) are spaced at unequal angles (thetai . . . , n) which can vary in percentage (theta%) from 0.5% to 10%, compared to the configuration with equal spacing angles (theta=) for fans with an equal number of blades. Preferably, the blades (4; 34) are delimited by a convex edge (7; 37), whose projection onto the rotation plane of the fan is defined by a parabolic segment and a concave edge (8; 38) whose projection onto the rotation plane of the fan is defined by a circular arc.

CROSS REFERENCE TO RELATED APPLICATION

The present application is the national stage under 35 U.S.C. 371 ofPCT/IB99/00458, filed Mar. 18, 1999.

TECHNICAL FIELD

The present invention relates to an axial flow fan for moving airthrough a heat exchanger and is preferably for use in the cooling andheating systems of motor vehicles.

Fans of this type must meet certain requirements, among which: low noiselevel, high efficiency, compact dimensions and ability to obtain goodvalues of pressure head and delivery.

BACKGROUND ART

Patent EP-0 553 598 B in the name of the same Applicant as the present,discloses a fan with blades having equal spacing angles. The blades havea constant chord length along their entire length and they are delimitedat the leading and trailing edges by two curves which, when projectedonto the plane of rotation of the fan wheel, are two circular arcs.

Although fans made in accordance with this patent achieve good resultsin terms of efficiency and low sound pressure, the sound distribution ofthe noise may be irritating to the human ear.

In fact, with the blades spaced at equal angles, there are cases ofresonance with a main harmonic whose frequency is the product of thenumber of revolutions per second of the fan wheel multiplied by thenumber of blades. This resonance gives rise to a hissing noise which maybe irritating to the human ear.

Even if the perception of irritation caused by a sound is mainlysubjective, there are basically two reasons which influence the noisedisturbance: the degree of sound pressure, that is, the intensity of thenoise and how it is distributed in terms of tone. As a result, lowintensity noises can also become irritating if the tone distribution ofthe noise distinguishes it from background noises.

To solve this problem, fans with blades spaced at unequal angles havebeen made.

Calculating an average of the sound intensity values at variousfrequencies, with the blades spaced at unequal angles the noise producedis almost equal to that with the blades spaced at equal angles. However,the different tone distribution of the noise allows an improvement inthe acoustic comfort. However, the fans with the blades spaced atunequal angles have a number of disadvantages.

The first disadvantage is the fact that in many cases the efficiency ofthe fans with blades spaced at unequal angles is less than that of thefans with spaced blades of equal angles.

Another disadvantage is the fact that the fan wheel with blades spacedat unequal angles may be unbalanced.

DISCLOSURE OF THE INVENTION

The aim of the present invention is to provide an improved axial fanwith a very low noise level.

Another aim of the present invention is to provide an improved axial fanwith good efficiency, head and delivery values.

Yet another aim of the present invention is to provide an improved axialfan whose fan wheel is substantially balanced naturally.

In accordance with an aspect of the present invention, an axial fan isdisclosed as specified in the independent claim. The dependent claimsrefer to preferred, advantageous embodiments of the invention.

The invention will now be described with reference to the accompanyingdrawings, which illustrate preferred embodiments of it, withoutrestricting the scope of the inventive concept, and in which:

FIG. 1 shows a front view of an embodiment disclosed in this invention.

FIG. 2 illustrates in a front view the geometrical features of a bladein some of the embodiments of the fan disclosed by the presentinvention;

FIG. 3 shows sections of a fan blade in some of the embodiments of thisinvention taken at regular intervals starting from the hub to the end ofthe blade;

FIG. 4 illustrates in a perspective view other geometrical features of ablade of some of the embodiments of the fan disclosed by this invention;

FIG. 5 shows a scaled-up detail of a part of the wheel and the relatedduct in some of the embodiments of this invention;

FIG. 6 is a front view of another embodiment of the present invention;

FIG. 7 shows a diagram representing, in Cartesian coordinates, theconvex edge of a fan blade in some of the embodiments of the presentinvention;

FIG. 8 is a diagram showing the changes in the blade angle in differentsections of a blade as a function of the radius of the fan in some ofthe embodiments of this invention;

FIG. 9 is a front view of another embodiment of this invention; and

FIG. 10 shows a schematic front view which defines the spacing angles ofthe blades in some embodiments of this invention.

The terms used to describe the fan are defined as follows:

the chord (L) is the length of the straight-line segment subtended bythe arc extending from the leading edge to the trailing edge over anaerodynamic profile of the section of the blade obtained by intersectingthe blade with a cylinder whose axis coincides with the axis of rotationof the fan and whose radius r coincides at a point Q;

the centre line or midchord line (MC) of the blade is the line joiningthe midpoints of the chords L to the different rays;

the sweep angle (δ) measured at a given point Q of a characteristiccurve of the blade, for example, the curve representing the trailingedge of the fan blade, is the angle made by a ray emanating from thecentre of the fan to the point Q concerned and the tangent to the curveat the same point Q;

the skew angle or net angular displacement (α) of a characteristic curveof the blade is the angle between the ray passing through thecharacteristic curve, for example, the curve representing the centreline or the midchord line of the blade, to the fan hub, and the raypassing through the characteristic curve at the end of the blade;

the blade spacing angle (θ) is the angle measured at the centre ofrotation between the rays passing through the corresponding points ofeach blade, for example, an edge at the end of the blades;

the blade angle (β) is the angle between the plane of rotation of thefan and the straight line joining the leading edge to the trailing edgeof the aerodynamic profile of the blade section;

the pitch ratio (P/D) is the ratio between the pitch of the helix, thatis to say, the amount by which the point Q concerned is axiallydisplaced, that is, P=2·π·r· tan (β), where r is the length of the rayto the point Q and β is the blade angle at the point Q and the maximumdiameter of the fan;

the profile camber (f) is the longest straight-line segmentperpendicular to the chord L, measured from the chord L to the bladecamber line; the position of the profile camber f relative to the chordL may be expressed as a percentage of the length of the chord itself;

the rake (V) is the axial displacement of the blade from the plane ofrotation of the fan, including not only the displacement of the entireprofile from the plane of rotation but also the axial component due tothe blade curvature, if any—also in axial direction.

With reference to the acompanying drawings, the fan 1 rotates about anaxis 2 and comprises a central hub 3 mounting a plurality of blades 4curved in the plane of rotation XY of the fan 1. The blades 4 have aroot 5, an end 6 and are delimited by a convex edge 7 and a concave edge8.

Since satisfactory results in terms of efficiency, noise level and headhave been obtained by rotating the fan made according to the presentinvention either in one direction or the other, the convex edge 7 andthe concave edge 8 may each be either the leading edge or the trailingedge of the blade.

In other words, the fan 1 may rotate in such a way that the air to bemoved meets first with the convex edge 7 and then the concave edge 8 or,vice versa, first with the concave edge 8 and then the convex edge 7.

Obviously, the aerodynamic profile of the blade section must be orientedaccording to the mode of operation of the fan 1, that is to say,according to whether the air to be moved meets the convex edge 7 or theconcave edge 8 first.

At the end 6 of the blades 4, a reinforcement ring 9 may be fitted. Thering 9 strengthens the set of the blades 4 for example by preventing theangle β of the blade 4 from varying in the area at the end of the bladeon account of aerodynamic loads. Moreover, the ring 9, in combinationwith a duct 10, limits the whirling of the air around the fan andreduces the vortices at the end 6 of the blades 4, these vortices beingcreated, as is known, by the different pressure on the two faces of theblade 4.

For this purpose, the ring 9 has a thick lip portion 11, that fits intoa matching seat 12 made in the duct 10. The distance (a), very small inthe axial direction, between the lip 11 and the seat 12 together withthe labyrinth shape of the part between the two elements, reduces airwhirl at the end of the fan blades.

Moreover, the special fit between the outer ring 9 and the duct 10allows the two parts to come into contact with each other while at thesame time reducing the axial movements of the fan.

As a whole, the ring 9 has the shape of a nozzle, that is to say, itsinlet section is larger than the section through which the air passes atthe end of the blades 4. The larger suction surface keeps air flowing ata constant rate by compensating for flow resistance.

However, as shown in FIG. 6, the fan made according to the presentinvention need not be equipped with the outer reinforcement ring and therelated duct.

The blade 4, projected onto the plane of rotation XY of the fan 1, hasthe geometrical characteristics described below.

The angle at the centre (B), assuming as the centre the geometricalcentre of the fan coinciding with the axis of rotation 2 of the fan,corresponding to the width of the blade 4 at the root 5, is calculatedusing a relation that takes into account the gap that must exist betweentwo adjacent blades 4. In fact, since fans of this kind are madepreferably of plastic using injection moulding, the blades in the dieshould not overlap, otherwise the die used to make the fan has to bevery complex and production costs inevitably go up as a result.

Moreover, it should be remembered that, especially in the case of motorvehicle applications, the fans do not work continuously because a lot ofthe time that the engine is running, the heat exchangers to which thefans are connected are cooled by the air flow created by the movement ofthe vehicle itself. Therefore, air must be allowed to flow througheasily even when the fan is not turning. This is achieved by leaving arelatively wide gap between the fan blades. In other words, the fanblades must not form a screen that prevents the cooling effect of theairflow created by vehicle motion. The relation used to calculate theangle (B) in degrees is:

B=(360°/No. of blades)−K; K _(min)=ℑ (hub diameter; height of bladeprofile at the hub).

The angle (K) is a factor that takes into account the minimum distancethat must exist between two adjacent blades to prevent them fromoverlapping during moulding and is a function of the hub diameter: thelarger the hub diameter is, the smaller the angle (K) can be. The valueof the angle (K) may also be influenced by the height of the bladeprofile at the hub.

The description below, given by way of example only and withoutrestricting the scope of the inventive concept, refers to an embodimentof a fan made in accordance with the present invention. As shown in theaccompanying drawings, the fan has seven blades, a hub with a diameterof 140 mm and an outside diameter, corresponding to the diameter of theouter ring 9, of 385 mm.

The angle (B), corresponding to the width of a blade at the hub,calculated using these values, is 44°.

The geometry of a blade 4 of the fan 1 will now be described: the blade4 is first defined as a projection onto the plane of rotation XY of thefan 1 and the projection of the blade 4 onto the plane XY is thentransferred into space.

With reference to the detail shown in FIG. 2, the geometricalconstruction of the blade 4 consists in drawing the bisector 13 of theangle (B) which is in turn delimited by the ray 17 on the left and theray 16 on the right. A ray 14, rotated in anticlockwise direction by anangle A=3/11 B relative to the bisector 13, and a ray 15, also rotatedin anticlockwise direction by an angle (A) but relative to the ray 16,are then drawn. The two rays 14, 15 are thus both rotated by an angleA=3/11 B, that is, A=12°.

The intersections of the rays 17 and 16 with the hub 3 and theintersections of the rays 14 and 15 with the outer ring 9 of the fan (orwith a circle equal in diameter to the outer ring 9), determine fourpoints (M, N, S, T) lying in the plane XY, which define the projectionof the blade 4 of the fan 1. The projection of the convex edge 7 is alsodefined, at the hub, by a first tangent 21 inclined by an angle C=3/4 A,that is, C=9°, relative to the ray 17 passing through the point (M) atthe hub 3.

As can be seen in FIG. 2, the angle (C) is measured in a clockwisedirection relative to the ray 17 and therefore the first tangent 21 isahead of the ray 17 when the convex edge 7 is the first to meet the airflow, or behind the ray 17 when the convex edge 7 is the last to meetthe air flow, that is, when the edge 8 is the first to meet the airflow.

At the outer ring 9, the convex edge 7 is also defined by a secondtangent 22 which is inclined by an angle (W) equal to 6 times the angle(A), that is, 72°, relative to the ray 14 passing through the point (N)at the outer ring 9. As shown in FIG. 2, the angle (W) is measured in ananticlockwise direction relative to the ray 14 and therefore the secondtangent 22 is ahead when the convex edge 7 is the first to meet the airflow, or behind the ray 14 when the convex edge 7 is the last to meetthe air flow, that is, when the edge 8 is the first to meet the airflow.

In practice, the projection of the convex edge 7 is tangent to the firsttangent 21 and to the second tangent 22 and is characterised by a curvewith a single convex portion, without points of inflection. The curvewhich defines the projection of the convex edge 7 is a parabola of thetype:

y=a x ² +b x+c.

In the embodiment illustrated, the parabola is defined by the followingequation:

y=0.013 x ²−2.7 x+95.7.

This equation determines the curve illustrated in the Cartesian diagram,shown in FIG. 7, as a function of the related x and y variables of theplane XY.

Looking at FIG. 2 again, the endpoints of the parabola are defined bythe tangents 21 and 22 at the points (M) and (N) and the zone of maximumconvexity is that nearest the hub 3.

Experiments have shown that the convex edge 7, with its parabolicprojection onto the plane of rotation XY of the fan, provides excellentefficiency and noise characteristics.

As regards the projection of the concave edge 8 of the blade 4 onto theplane XY, any second-degree curve arranged in such a way as to define aconcavity can be used. For example, the projection of the concave edge 8may be defined by a parabola similar to that of the convex edge 7 andarranged in substantially the same way.

In a preferred embodiment, the curve defining the projection of theconcave edge 8 onto the plane XY is a circular arc whose radius (R_(cu))is equal to the radius (R) of the hub and, in the practical applicationdescribed here, the value of this radius is 70 mm.

As shown in FIG. 2, the projection of the concave edge 8 is delimited bythe points (S) and (T) and is a circular arc whose radius is equal tothe radius of the hub. The projection of the concave edge 8 is thuscompletely defined in geometrical terms.

FIG. 3 shows eleven profiles 18 representing eleven sections of theblade 4 made at regular intervals from left to right, that is, from thehub 3 to the outer edge 6 of the blade 4. The profiles 18 have somecharacteristics in common but are all geometrically different in orderto be able to adapt to the aerodynamic conditions which aresubstantially a function of the position of the profiles in the radialdirection. The characteristics common to all the blade profiles areparticularly suitable for achieving high efficiency and head and lownoise.

The first profiles on the left are more arched and have a larger bladeangle (β) because, being closer to the hub, their linear velocity isless than that of the outer profiles .

The profiles 18 have a face 18 a comprising an initial straight-linesegment. This straight-line segment is designed to allow the airflow toenter smoothly, preventing the blade from “beating” the air which wouldinterrupt smooth airflow and thus increase noise and reduce efficiency.In FIG. 3, this straight-line segment is labelled (t) and its length isfrom 14% to 17% of the length of the chord (L).

The remainder of the face 18 a is substantially made up of circulararcs. Passing from the profiles close to the hub to wards those at theend of the blade, the circular arcs making up the face 18 a becomelarger and larger in radius, that is to say, the profile camber (f) ofthe blade 4 decreases.

With respect to the chord (L), the profile camber (f) is located at apoint, labelled (1 f) in FIG. 3, between 35% and 47% of the total lengthof the chord (L). This length must be measured from the edge of theprofile that meets the air first.

The back 18 b of the blade is defined by a curve such that the maximumthickness (G_(max)) of the profile is located in a zone between 15% and25% of the total length of the blade chord and preferably at 20% of thelength of the chord (L). In this case too, this length must be measuredfrom the edge of the profile that meets the air first.

Moving from the profiles closer to the hub where the maximum thickness(G_(max)) has its highest value, the thickness of the profile 18decreases at a constant rate to wards the profiles at the end of theblade where it is reduced by about a quarter of its value. The maximumthickness (G_(max)) decreases according to substantially linearvariation as a function of the fan radius. The profiles 18 of thesections of the blade 4 at the outermost portion of the fan 1 have thelowest (G_(max)) thickness value because their aerodynamiccharacteristics must make them suitable for higher speeds. In this way,the profile is optimised for the linear velocity of the blade section,this velocity obviously increasing with the increase in the fan radius.

The length of the chord (L) of the profiles (18) also varies as afunction of the radius.

The chord length (L) reaches its highest value in the middle of theblade 4 and decreases to wards the end 6 of the blade so as to reducethe aerodynamic load on the outermost portion of the fan blade and alsoto facilitate the passage of the air when the fan is not operating, asstated above.

The blade angle (β) also varies as a function of the fan radius. Inparticular, the blade angle (β) decreases according to a quasi-linearlaw.

The law of variation of the blade angle (β) can be chosen according tothe aerodynamic load required on the outermost portion of the fan blade.

In a preferred embodiment, the variation of the blade angle (β) as afunction of the fan radius (r) follows a cubic law defined by theequation

(β)=−7·10⁻⁶ ·r ³+0.0037·r ²−0.7602r+67.64

The law of variation of (β) as a function of the fan radius (r) isrepresented in the diagram shown in FIG. 8.

FIG. 4 shows how the projection of the blade 4 in the plane XY istransferred into space. The blade 4 has a rake V relative to the planeof rotation of the fan 1.

FIG. 4 shows the segments joining the points (M′, N′) and (S′, T′) of ablade (4).

These points (M′, N′, S′, T′) are obtained by starting from the points(M, N, S, T) which lie in the plane XY and drawing perpendicularsegments (M, M′), (N, N′), (S, S′), (T, T′) which thus determine a rake(V) or, in other words, a displacement of the blade 4 in axialdirection. Moreover, in the preferred embodiment, each blade 4 has ashape defined by the arcs 19 and 20 in FIG. 4. These arcs 19 and 20 arecircular arcs whose curvature is calculated as a function of the lengthof the straight-line segments (M′, N′) and (S′, T′). As shown in FIG. 4,the arcs 19 and 20 are offset from the corresponding straight-linesegments (M′, N′) and (S′, T′) by lengths (h1) and (h2) respectively.These lengths (h1) and (h2) are measured on the perpendicular to theplane of rotation XY of the fan 1 and are calculated as a percentage ofthe length of the segments (M′, N′) and (S′, T′) themselves.

The dashed lines in FIG. 4 are the curves—parabolic segment and circulararc—related to the convex edge 7 and to the concave edge 8.

The rake V of the blade 4, both as regards its axial displacementcomponent and as regards curvature makes it possible to correct bladeflexures due to aerodynamic load and to balance the aerodynamic momentson the blade in such a way as to obtain uniform axial air flowdistributed over the entire front surface of the fan.

All the characteristic values of the fan blade, according to theembodiment described, are summarised in the table below where r is thegeneric fan radius and the following geometrical variables refer to thecorresponding radius value:

L indicates the chord length;

f indicates the profile camber

t indicates the initial straight-line segment of the blade section;

1 f indicates the position of the profile camber relative to the chordL;

β indicates the angle of the blade section profile in sexagesimaldegrees;

x and y indicate the Cartesian co-ordinates in the plane XY of theparabolic edge of the blade.

r 70 100.6 131.2 161.9 179 L 59.8 68.7 78.2 73 71.2 f 8.2 7.5 7.8 6.7 5t 10 10.5 11 10.5 10 lf 21 25.5 31.2 32.8 33 β 30.1 21.9 15.7 13.3 11.1x 65.3 93.2 126.1 161.9 176.4 y −25.2 −43.0 −38.1 −0.7 23.9

Experiments comparing the conventional fans with those made inaccordance with the embodiments using blades spaced at an equal angle θ,show that there is a decrease in the sound power of about 25% to 30%,measured in dB(A) with an improvement in acoustic comfort.

Furthermore, under the same conditions of air delivery, the fans madeaccording to the embodiments with blades spaced at an equal angle θ,have developed head values up to 50% greater compared to theconventional fans of this type.

In fans made according to the embodiments, with blade s spaced at anequal angle θ, passing from a blades back to a blade s forwardconfiguration, there are no appreciable changes in noise level.Moreover, under certain working conditions of the fan, in particular inthe high head range, the blades forward configuration delivers 20-25%more than the blades back configuration.

FIGS. 9 and 10 show another embodiment of a fan 30 comprising a wheel 31with blades 34 spaced at unequal angles θ. The embodiment with blades ofunequal angles θ further improves the acoustic comfort. The differentnoise distribution from the fan made in accordance with this embodimentmakes it even more pleasant to the human ear.

With reference to FIGS. 9 and 10, the wheel 31 has seven blades 34positioned at the following angles, expressed in sexagesimal degrees:

θ1=55.381; θ2=47.129; θ3=50.727; θ4=55.225; θ5=50.527; θ6=48.729;θ7=52.282.

If the wheel 31 had the blades 34 spaced at equal angles or as the fansembodied in FIGS. 1 and 6, the spacing angle would be θ₌=360°/7=51.429°.

The table set out below shows the values of the unequal anglesθ_(i, . . . , n.) θ₌ and the absolute and percentage deviations of thevalues of the unequal angles θ_(i, . . . , n) compared to thecorresponding value of the equal angle θ₌ for fans with seven blades:

number of blades 7 blades with deviation % unequal blades with(θ_(i, . . . n) −θ₌) angles equal angles deviations -----------100angles (θ_(i . . . n)) (θ₌) (θ_(i, . . . n) −θ₌) θ_(n) θ1 55.381 51.4293.952 7.685 θ2 47.129 51.429 −4.300 −8.360 θ3 50.727 51.429 −0.702−1.364 θ4 55.225 51.429 3.796 7.382 θ5 50.527 51.429 −0.902 −1.753 θ648.729 51.429 −2.700 −5.249 θ7 52.282 51.429 0.853 1.659 TOTAL 360° 360°0.00 0.00

More precisely, the second column shows the values of the anglesθ_(i, . . . , n,) in accordance with the present embodiment; the thirdcolumn shows the values of the angles θ₌ when all angles are equal; thefourth column shows the algebraic difference or algebraic deviationbetween the values of the angles of the second and third column; thefifth column shows the value of the deviation of the fourth columnexpressed as a percentage of the angles in the third column θ₌.

The table shows that the percentage and algebraic deviation in theangles are relatively low compared to the configuration of blades spacedat equal angles. According to the present embodiment, the values of thepercentage deviation of the blade spacing angles should be between 0.5%and 10%.

Hence, even if an improvement in noise characteristics is achieved, theefficiency of the wheel with the blades spaced at equal angles issubstantially the same.

As can be seen in more detail below, if the deviation percentage valuesare maintained within these limits, wheels which are substantiallybalanced can be made even with any number of blades n greater thanthree, and therefore different from the wheel 31 which has seven bladesas shown in the example. Even the embodiments made with a number ofblades 34 other than seven and with those limitations regarding angularspacing achieve good results in terms of efficiency and noise level.

The noise produced by the fans made with the angles θ_(i . . . , n)mentioned above has almost the same intensity but is less irritating tothe human ear. A good result was achieved regarding the pleasantness ofthe noise in the configuration with the blade s forward and theconfiguration with the blades back.

Preferably, the configuration of the blades 34 mentioned above can beused in combination with the blades 4 with a parabolic edge 7 of otherembodiments previously mentioned. Also in this case, the values of head,delivery and efficiency are substantially invariable.

Another advantage of this configuration is that the centre of gravity isalways on the rotation axis 32 of the fan 30. In analytical termsconsidering a reference system whose origin is on the rotation axis, thefollowing is true:${X_{g} = {\frac{\sum\quad {m_{i}*x_{i}}}{\sum\quad m_{i}} = 0}};$$Y_{g} = {\frac{\sum\quad {m_{i}*y_{i}}}{\sum\quad m_{i}} = 0.}$

where the X_(g) and Y_(g) are the Cartesian co-ordinates of the centreof gravity of the fan wheel 30 and m_(i) x_(i) y_(i) are the mass andthe Cartesian co-ordinates of the centre of gravity of each blade 34,respectively.

In the example, shown in FIGS. 9 and 10 of a wheel 31 with n blades ofequal mass m the formula is the following:${{X_{g}\frac{\sum\quad {m*x_{i}}}{m*n}} = 0};$${Y_{g}\frac{\sum\quad {m*y_{i}}}{m*n}} = 0.$

With this configuration a wheel 31 already substantially balancedwithout the need to intervene on the mass of the blades 34 can beachieved, or any such an intervention is reduced to the minimum comparedto that needed to balance the wheels of the type with have blades spacedat unequal angles. There are therefore advantages in terms of simple andeconomical construction.

What is claimed is:
 1. An axial flow fan (1; 30) having a geometricalcentre, rotating in a rotation plane (XY) about an axis (2) coincidingwith the geometrical centre of the fan (1), the fan (1) including acentral hub (3; 33), a plurality (n) of blades (4; 34) each having aroot (5; 35) and an end (6; 36), each blade (4; 34) being also delimitedby a convex edge (7) defined by a parabolic segment and a concave edge(8) defined by a second degree geometric curve, each blade (4; 34)consisting of blade sections with aerodynamic profiles (18), saidaerodynamic profiles (18) having a leading edge, a trailing edge andhaving a blade angle (β) which decreases gradually and constantly fromthe root (5) to wards the end (6) of the blade (4), the blade angle (β)being defined as the current angle between the rotation plane (XY) and astraight line joining the leading edge to the trailing edge of theaerodynamic profile (18) of each blade section, the blades (4; 34) beingspaced at unequal angles (θ_(i . . . , n)), the unequal spacing angles(θ_(i . . . , n)) varying in percentage (θ%) by values between 0.5% and10% compared to a configuration with equal spacing angles (θ₌) for fanswith the same number (n) of blades, that is: 0.5≦θ%≦10, where${{\theta \quad \%} = {\frac{\theta_{i,\ldots \quad,n} - \theta_{=}}{\theta_{=}} \cdot 100}},$

so that the fan (1; 30) is substantially balanced naturally.
 2. The fanaccording to claim 1 characterized in that each blade (4) projected ontothe rotation plane (XY) is delimited by four points (M, N, S, T) lyingin the plane (XY) and defined as a function of a blade width angle (B),said blade width angle (B) having a bisector (13), being subtended atthe centre of the fan, being defined by a first ray (17) and a secondray (16) emanating from the centre of the fan and corresponding to thewidth of a single blade (4) at the root (5); each blade (4) beingcharacterized also in that the four points (M, N, S, T) are determinedby the following geometric characteristics: the first point (M) islocated at the intersection of the hub (3) and the blade, or at theintersection of the root (5) of the blade (4) with the first ray (17)defining the blade width angle (B); the second point (S) adjacent to thefirst point (M) is located at the intersection of the hub (3) and theblade, or at the intersection of the root (5) of the blade (4) with thesecond ray (16) defining the blade width angle (B); the third point (N)is located at the end (6) of the blade (4) and is displaced in ananticlockwise direction by an advance angle (A)=3/11*(B) relative to thebisector (13) of the blade width angle (B); the fourth point (T)adjacent to the third point (N) is located at the end (6) of the blade(4) and is displaced in the anticlockwise direction by the advance angle(A)=3/11*(B) relative to the second ray (16) emanating from thegeometrical centre of the fan and passing through the second point (S).3. The fan according to claim 2 characterized in that the projection ofthe convex edge (7) onto the rotation plane (XY) at the first point (M)has a first tangent (21) inclined by a first tangent angle (C) equal tothree quarters of the advance angle (A) relative to the first ray (17)passing through the first point (M); and characterized also in that theprojection of the convex edge (7) onto the rotation plane (XY) at thethird point (N) has a second tangent (21) inclined by a second tangentangle (W) equal to six times the advance angle (A) relative to a thirdray (14) passing through the geometrical centre of the fan (1) and saidthird point (N); the first and second tangents (21, 22) being ahead ofthe respective first and third rays (17, 14) when the direction ofrotation of the fan (1) is such that the convex edge (7) corresponds tothe leading edge of the aerodynamic profile (18) of each blade sectionand the first and second tangents (21, 22) are arranged in such a way asto define a curve, in the rotation plane (XY), that has a single convexportion without flexions.
 4. The fan according to claim 1 characterizedin that it comprises seven blades (34) and in that the unequal spacingangles (θ_(1 . . , n)) of the blades (34) respectively have valuesexpressed in degrees of: 55.381; 47.129; 50.727; 55.225; 50.527; 48.729;52.282.
 5. The fan according to claim 1 characterized in that theprojection of the concave edge (8) onto the plane (XY) is defined by aparabolic segment.
 6. The fan according to claim 1 characterized in thatthe projection of the concave edge (8) onto the plane (XY) is defined bya circular arc.
 7. The can according to claim 6 characterised in thatthe circular arc formed by the projection of the concave edge (8) ontothe plane (XY) has a radius (R_(cu)) equal to the radius (R) of the hub(3).
 8. The fan according to claim 1 characterized in that theaerodynamic profiles (18) have a face (18 a) comprising at least oneinitial straight-line segment (t).
 9. The fan according to claim 8characterized in that the face (18 a) includes a segment, following theinitial segment (t), comprising portions of circular arcs.
 10. The fanaccording to claim 8 characterized in that the aerodynamic profiles (18)each have a chord length (L) and a back (18 b) defined by a convex curvewhich, in combination with the face (18 a), determines a maximumthickness value (G_(max)) of the profile in a zone between 15% and 25%of the total length of the chord (L) measured from the leading edge. 11.The fan according to claim 12 characterized in that the blades (4) areformed of sections whose aerodynamic profiles (18) each have a bladeangle (β) that decreases gradually and constantly from the root (5) towards the end (6) of the blade (4) according to a cubic law of variationas a function of the radius of the fan at which said sections arelocated.